Two-Electron HCl to H₂ Photocycle Promoted by Ni(II) Polypyridyl Halide Complexes

Abstract

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp2)–C(sp3)Coupling Reactions

“…Biphenyl was also observed as a product. Although attempts to crystallize 4 were unsuccessful, the 1 H NMR spectrum is similar to the previously reported homoleptic complex [(neocuproine) 2 Ni I ](PF 6 ), and the EPR spectrum is consistent with a metal‐centered radical with axial geometry. Mass spectrometry indicates the presence of a cationic, [(neocuproine) 2 Ni I ] unit.…”

“…Similar electronic configurations were observed in terpyridine‐ligated Ni I complexes . This configuration, however, is different to the only other known bidentate polypyridyl‐containing Ni I species, which are cationic homoleptic complexes of phenanthroline derivatives that contain a metal‐centered radical . It is also intriguing that 1 is formed through a reaction with a common sp 2 ‐electrophile, as it raises the possibility that previously unobserved dinuclear species could be present in catalysis.…”

Alkyl Bromides as Mild Hydride Sources in Ni-Catalyzed Hydroamidation of Alkynes with Isocyanates

“…As shown, 3a-D was prepared in high yields from either (CD 3 ) 2 CHBr or (CH 3 ) 2 CDBr with 75% and 15%-D content, respectively, suggesting that a b-hydride elimination/migratory insertion might occur at some extent prior to alkyne binding. 22 The stoichiomeric experiments with Ni(L4) 2 are particularly illustrative (Scheme 3, bottom); 23 specifically, we found that 3a was solely obtained in the presence of i-PrBr, thus ruling out an oxidative cyclization of Ni(0), 1a and 2a en route to I. 6 Although further studies are needed, we currently propose a pathway consisting of a hydrometalation with in situ generated Ni(II)-hydride, single electron transfer (SET) mediated by Mn en route to a Ni(I) intermediate 24 followed by RNCO insertion 25 and a final SET to recover the Ni(0) species.…”

Nickel‐Catalyzed Carboxylation of Benzylic C−N Bonds with CO2

“…Although a comprehensive study detailing the mechanistic underpinnings of this reaction will require further investigations, we decided to study the reactivity of Ni 0 ( L4 ) 2 ( 5 ) and Ni I ( L4 ) 2 compounds ( 6 ). While 18‐electron complex 5 was prepared in quantitative yield by reacting Ni(COD) 2 and L4 in benzene at 40 °C, complex 6 was prepared from 5 upon exposure to AgOTf in THF at RT . As shown in Scheme , both structures were unambiguously characterized by X‐ray crystallography .…”